1. Field of the Invention
The present invention relates to a magnetic storage device or the like that stores data in magneto-resistive elements.
2. Description of the Related Art
MRAM (Magnetic Random Access Memory) has been receiving much attention recently as a memory device to be used in information processing equipment such as computers, communication equipment, and the like. Since MRAM stores data utilizing magnetism, it is capable of retaining the direction of magnetization without use of any electrical means, and hence, data are not inconveniently lost when power is cut, as is the case with DRAM (Dynamic Random Access Memory) or SRAM (Static RAM), which are generally referred to as volatile memories. Furthermore, when compared with conventional nonvolatile storing means such as flash EEPROM and hard disc drives, MRAM excels in performance in terms of access speed, reliability, power consumption, and the like. Because of these characteristics, it is a common belief that MRAM will be able to simultaneously realize all the advantages of not only volatile memories such as DRAMs and SRAMs but also of nonvolatile memories such as flash EEPROMs and hard disc drives.
Consider, for example, the case of developing information processing equipment that targets so-called ubiquitous computing in which information processing is available regardless of location. A requirement of such ubiquitous computing includes a memory device that must be capable of high speed processing but simultaneously have low power consumption. Such a memory device must also be able to avoid loss of information, even upon power shutdown. MRAM has the potential to meet both of these demands, and is expected to be employed in an increasing number of information processing equipment designs in the future.
In the case of tablets, mobile information terminals, and the like that are carried by a person in day-to-day living, it is often difficult to secure a sufficient power supply. Therefore, in order to be able to process a large amount of information in a very high usage environment, even MRAMs of low power consumption will require a further reduction in power consumption during information processing.
One example of the technology that improves the power consumption rate of an MRAM is a magnetic storage device described as follows. As shown in FIG. 21, this magnetic storage device 500 includes, in each of the storage areas (memory cells), a bit line 502, a word line 504 disposed orthogonal to the bit line 502, and a tunneling magneto-resistive (TMR) element 506 disposed at the intersection between the bit line 502 and the word line 504. Each of the bit line 502 and the word line 506 is capable of creating a magnetic field whose strength is approximately half that necessary to invert the bit state of the TMR element 506. When a current flows through the bit line 502 and the word line 504 that have been selected, the TMR element 506 at the cross-point inverts its magnetization configuration accordingly.
In this magnetic storage device 500, the bit line 502 and the word line 504 both have a cladding structure where they are coated with a ferromagnetic film 510 which exhibits high magnetic permeability. Therefore, any leak of magnetic flux from the bit line 502 and the word line 504 can be reduced. Furthermore, when the bit line 502 or the word line 504 is energized, the ferromagnetic film 510 becomes magnetized, thereby creating a static magnetic field. Therefore, the sum of this static magnetic field and induced magnetic fields of the bit line 502 and the word line 504 are applied onto the TMR element 506. As a result, even if the power supply is low, the magnetic field that is necessary to invert the magnetization configuration of the TMR element 506 can still be obtained.
Moreover, by coating three surfaces of the bit line 502 and the word line 504, respectively, with the ferromagnetic film 510 but leaving the remaining surface facing the TMR element 506 side open, magnetic flux can be concentrated onto the TMR element 506. This has an advantage in that a write cycle would require less time to be completed.
It should be noted that the TMR element in this instance includes a first magnetic layer (magnetic sensing layer) whose direction of magnetization changes according to an external magnetic field, a second magnetic layer whose direction of magnetization is fixed, and a nonmagnetic insulating layer interposed between the first magnetic layer and the second magnetic layer. This TMR element stores binary data by controlling the orientation of the magnetization directions of the first and second magnetic layers, so that the direction of magnetization is either parallel or antiparallel.
The technology of this magnetic storage device 500 is disclosed by the following documents.
Non-Patent Document 1: Nikkei Electronics, p. 133, Nov. 18, 2002.
Non-Patent Document 2: M. Durlam, et al, “A 1-Mbit MRAM Based on 1T1MTJ Bit Cell Integrated with Copper Interconnects,” IEEEJ Solid-State Circuits 38, 769 (2003)
Non-Patent Document 3: Hiromi Niu Fuke, et al, “Spin-valve giant magnetoresistive films with ant ferromagnetic Ir—Mn layers”, JAP 81, 4004 (1997)
Non-Patent Document 4: K Hoshino, et al, “Exchange Coupling between Antiferromagnetic Mn—Ir and Ferromagnetic Ni—Fe Layers”, JJAP 35, 607 (1996)
According to further research carried out by the inventor of the present invention, however, although the coating of these bit lines 502 and word lines 504 with the ferromagnetic film 510 can reduce the current during write cycles, it is likely that it will make the strength of the resultant magnetic field uneven. In particular, it is difficult to evenly coat the bit line 502 or the word line 504 with the ferromagnetic film 510 along its longitudinal direction. In addition to this, a plurality of domains whose magnetization directions vary widely will be spontaneously formed within the ferromagnetic film 510. These factors may contribute to the possibility that magnetization characteristics acting on respective TMR elements 506 during a write cycle may become uneven.
In addition, when a magnetic field is being inverted by switching the direction of current flowing through the bit line 502 or the word line 504, the presence of the ferromagnetic film 510 causes a problem in that the strength or the rate of change of the magnetic field becomes uneven, depending on which way the current is flowing. As a result, each TMR element 506 experiences an unevenness in write speed, depending on the direction of the current, and it is a concern that control of the current or timing during the write cycle may become complicated.
Furthermore, if many domains are formed within the ferromagnetic film 501, Barkhausen noise is produced when the magnetization configuration of the bit line 502 or the word line 504 changes, and this is also considered to contribute to the deterioration of the write cycle.